It was unclear who carried out the attack on the car, he said, and police had been given conflicting accounts of what happened.

The Kenyan Red Cross said four people had been killed in heavy machine-gun fire on a small car heading from Mandera to a school.

Mandera is in a part of Kenya which suffers from chronic insecurity and both banditry and incursions by militant groups are relatively common.

Earlier this week, Kenya was shaken by twin grenade attacks in the capital, Nairobi -- one at a nightclub and a second at a crowded downtown bus station during rush hour -- which left one person dead and 20 injured.

A Kenyan man pleaded guilty Wednesday to being a member of the Islamic militant group Al-Shabaab and was charged with causing grievous bodily harm in the bus station attack.

It is not clear if Thursday's vehicle attack is linked to Monday's violence.

The U.S. Embassy in Kenya warned last week it had credible information regarding an imminent terror attack, but offered no details on who might carry out such an attack.

Kenya has been on edge since it sent troops across the border into Somalia nearly two weeks ago to pursue militants with Al-Shabaab, an Islamist group that the United States and other countries consider a terrorist organization. Kenya's action followed the recent abductions of tourists and aid workers in Kenya. It blames the abductions on Al-Shabaab, which has denied involvement.

Al-Shabaab has threatened to attack Kenya if it does not withdraw its forces from Somalia.